It is known to produce ion exchange membranes for use in electrolytic cells or fuel cells. The ion exchange membranes partition the cathode compartment from the anode compartment and do not substantially pass the electrolyte and selectively pass ions.
Ion exchange membranes are conventionally fabricated from fluorinated polymers with carboxylic acid groups or sulfonic acid groups as the ion exchange group. The present invention deals only with the production of fluorinated polymers having sulfonic acid groups.
The fluoropolymer containing sulfonic acid groups has excellent heat resistance and chemical resistance (alkali resistance, acid resistance, chlorine resistance, etc.). For example, the fluoropolymer is substantially inactive to the electrolysis and the electrolyzed products in an alkali electrolysis and further has substantially impermeability to liquid and gas, which makes possible the long stable operation.
Membranes made from sulfonic-acid type fluorinated copolymers should have physical integrity and strength commensurate with the physical demands of the device in which they will be used. In electrochemical devices, such as electrolytic cells, physical demands on the membrane vary, depending upon the type of cell and the configuration of the cell. For example, in some cells, electrodes (anodes and cathodes) are spaced substantially apart from each other and have the membrane placed between the two electrodes. In such cell configurations, the membrane functions, more or less, as a free-standing film. Such free-standing membrane films are commonly reinforced to increase their strength. Reinforcement materials which are commonly used include a variety of materials in the form of woven scrims and randomly-dispersed fibers. However, even when supported, the membrane must still have certain minimum levels of physical integrity. Otherwise, it breaks apart and looses its utility.
Physical integrity of ionic fluoropolymers is determined, to a large degree, by the amount of water or solvent the fluoropolymers contain. Thus, a sulfonic fluoropolymer that swells excessively because it absorbs substantial amounts of water or solvent tends to become gel-like and loses much of its physical integrity, relative to an unswollen sulfonic fluoropolymer. The level of swelling (the level of water absorption) for a particular ionic fluoropolymer is also determined by the temperature and the environment. The physical integrity of the membrane may be predicted by measuring its melt flow; i.e., membranes with low melt flow have greater physical integrity.
Another matter of concern in defining usefulness of sulfonic fluoropolymers as membranes is the chemical requirements in a given application. For example, a membrane formed from a sulfonic fluoropolymer in a chloralkali cell has two critical criteria that it should preferably satisfy: electrical conductivity and the ability to reject anions. The sulfonic fluoropolymer chosen for use in such conditions is usually based on a trade-off between the electrical conductivity of the polymer, which is effected by both equivalent weight and water absorption, and the polymer's ability to reject hydroxide ions, which is largely determined by the level of hydration, i.e., the degree of hydration per functional group in the sulfonic fluoropolymer. Under these circumstances, where it is desired to minimize the membrane's passage of hydroxide ions, one chooses a sulfonic fluoropolymer having a higher equivalent weight than a membrane designed to maximize electrical conductivity. Thus, the melt flow of the fluoropolymer may not be the only deciding factor in choosing the fluoropolymer for this particular use.
In general, when the ion exchange capacity is increased, the molecular weight of the fluorinated polymer has been lowered which compromises the physical strength of a film or membrane. Therefore, it would be very advantageous to have fluoropolymers which have high ionic conductivity while yet maintaining physical integrity.
Sulfonic fluoropolymers having equivalent weights less than about 800 are generally useful as films in a variety of electrochemical applications. Below an equivalent weight of about 800, water absorption of the membrane increases and physical integrity of the membrane decreases. Sulfonic fluoropolymers having equivalent weights less than about 750, but usually not less than about 500, are generally useful in applications where ionic conductivity is the prime concern and physical requirements are minimal.
Sulfonic acid copolymers having the desired characteristics may be prepared by polymerization of TFE with various perfluoroolefins containing sulfonic functional groups. Copolymerization of TFE with various perfluoroolefins containing sulfonic functional groups to form polymers which are useful as films or membranes in electrolytic cells is well known in the art. Prior art polymerization methods include solution, aqueous, dispersion, bulk, thermal, and radiation-induced polymerization.
The prior art mentions polymerization of TFE and comonomer (A). For example, the water absorption of a membrane formed from a copolymer of TFE and comonomer (A) having an equivalent weight 1000 in the --SO.sub.3 H form which has been boiled in water for 30 minutes is about 45%. The physical integrity of this membrane is poor, especially in the harsh environment of a chloralkali electrolytic cell.
The prior art mentions emulsion polymerization to make a fluoropolymer of TFE and comonomer (B). One reference discloses a copolymer of TFE and comonomer (B) with a ratio of about 3.6 to 1. This polymer has a water absorption of 95%, which is very high compared with that of commercial TFE copolymers, causing the membrane to be tough and elastic. The physical integrity of this membrane is poor. A film at equivalent weight 564, produced from 2.86 to 1 TFE/(B), has a water absorption after boiling in water for 30 minutes of 646%, which results in very low modulus and very poor physical integrity.
The copolymerization of TFE and comonomer (B) by solution (nonaqueous) polymerization has been mentioned but never exemplified in the prior art. At low ratios of TFE to comonomer (B), copolymers made by other polymerization methods have shown very high water absorption.
It is desired to have a film of low melt flow and high molecular weight to provide good physical properties such as durability, tear strength and high modulus. This is particularly desirable for unreinforced films used in fuel cells. Until the present invention, low melt flow and high molecular weight was attainable only at high equivalent weight (which is achieved by using high TFE/(A) or TFE/(B) ratios). These high TFE ratios cause undesirably high ionic resistance. This is a particular disadvantage for fuel cell membranes. It is also desirable to have low resistance, low stickiness, and good tear strength when a fluoropolymer of TFE/(A) or TFE/(B) is used as one or more layers of a membrane in a chloralkali cell.
The present invention provides an advantageous process for producing a fluorinated polymer having sulfonic-acid type ion exchange groups, especially a fluorinated polymer having high molecular weight, high ion exchange capacity, good durability, high strength, low melt flow and low stickiness.
As the result, it has been found that a desired fluorinated polymer can be obtained by copolymerizing a fluorinated olefin and a fluorinated monomer having sulfonic-acid type functional group with or without an inert organic solvent.